Make sure you are playing with 3 other real people if you're trying to get that specific achievement @Gangstatron. The computer controlled survivors will shoot you over and over trying to hit something behind you.

i met 3 of my "best" xbox friends by joining random games. I even made a real friend playing L4d with randoms. We figured out we lived 20 minutes apart and we went to the gun range together.

I've had some sh!tty experiences with randoms too. One time, while going for expert all alone, some random joined in the last map. I made it by myself all the way there now this guy is running for the finish threatening to end it all for me. He had has friend joint and try to kick me, luckily my friend joined at the same time and blocked them. We sat there shooting each other at a stale mate. They wouldn't leave. I wouldn't give up after 3-4 hours of playing. They eventually left, we blocked them. And got the expert. Fuckings ramdoms.